Process for production of titanium alloy

ABSTRACT

Titanium-aluminum alloy is prepared as a master alloy, and the aluminum master alloy and a pure titanium material are melted by an electron beam to yield titanium alloy.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to a process for production oftitanium alloy, and in particular, relates to a process for productionof Ti-Al alloy using an intermetallic compound of titanium-aluminum.

[0003] 2. Background Art

[0004] Recently, titanium materials have been used not only forairplanes but also for general uses. In particular, titanium alloys arewidely used in fields in which corrosion resistance or weight reductionis required.

[0005] However, titanium alloys are not widely used because they areexpensive compared to other materials. In particular, although Ti-6 wt %Al-4 wt % V alloy exhibits superior strength and corrosion resistance,it has not found wide consumer use due to its high cost.

[0006] In Japanese Unexamined Patent Application Publication No.158955/92, a technique in which Ti-6 wt % Al-4 wt % V alloy having a lowlevel impurities is produced at low cost by adding an excess amount ofpure Al with respect to the desired composition to titanium alloy scrapcontaining Al and melting the material by EB (electron beam) melting isdisclosed. However, in this technique, since an excess amount of Al isadded due to vaporization of Al in the EB melting process, the amount ofvaporized Al varies and control of the alloy composition is difficult.Furthermore, since Al is added in controlling the Al content, pure Al asa raw material must be formed into briquettes to weigh the amount ofadded Al, whereby producing cost may be increased.

[0007] In addition, demand for titanium material for a target isrecently increasing as electric materials are widely used. However,efficient recycling methods for spent target material are unknown.

SUMMARY OF THE INVENTION

[0008] Therefore, the present invention was completed in view of thesituation explained above. An object of the invention is to provide aprocess for production of Ti—Al alloy, which is inexpensive and reliablein quality.

[0009] The inventors have researched to solve the problems describedabove, and they have found that inexpensive Ti—Al alloy having lowcomponent variation can be produced by using titanium-aluminum alloy asa master alloy of the aluminum component, and melting the material in anEB furnace. The present invention is completed based on the aboveknowledge.

[0010] That is, the present invention provides a process for productionof titanium alloy comprising the steps of preparing titanium-aluminumalloy as a master alloy, and melting this aluminum master alloy and puretitanium material by an electron beam to obtain titanium alloy.

[0011] In the process for production mentioned above, sincetitanium-aluminum alloy having low vapor pressure is used as the masteralloy of the aluminum component, variation of aluminum content in thetitanium alloy obtained by electron beam melting is low, and the contentcan be reliable. Furthermore, since titanium-aluminum alloy can berelatively easy to obtain as scrap of titanium alloy containing high Al,producing cost can be reduced.

[0012] Preferable embodiments of the present invention are explainedbelow. Titanium-aluminum alloy is defined by a formula Ti_(x)Al, andsufficient effects can be exhibited in the case in which x is in a rangeof from ⅓ to 3 in the present invention. In the case in which excessamount of Al with respect to the above range is contained, Al lossduring melting is extreme and undesirable from the viewpoint ofcomposition control and yield efficiency. On the other hand, in the casein which Al is contained in an amount below the range, desired Ti-6Al-4Valloy composition cannot be maintained, and metal Al must be supplied.In this case, vaporizing loss of Al in the melting is also extreme andundesirable from the viewpoint of composition control.

[0013] Therefore, it is desirable that titanium-aluminum alloy having acomposition within the range be used as the aluminum source.

[0014] Alternatively, in the present invention, among titanium-aluminumalloys, a titanium-aluminum intermetallic compound can be used. Ti₃Al,TiAl, TiAl₂, TiAl₃ or the like can be used as the intermetalliccompound. In particular, among these intermetallic compounds, Ti₃Al andTiAl can reduce vaporizing loss in the melting because of their highvapor pressure.

[0015] It should be noted that not only can a single intermetalliccompound be used, but also a mixture of intermetallic compounds can beused as the aluminum source.

[0016] In addition, intermetallic compounds having compositions otherthan Ti₃Al, TiAl, TiAl₂, TiAl₃ can be used.

[0017] As a preferable example of a titanium alloy of the presentinvention, Ti—Al—V alloy, for example, Ti-6Al-4V, may be mentioned.Furthermore, the invention can be widely applied to alloys in which Alor V is contained as a main component, for example Ti-10V-2Fe-3Al alloy,Ti-6Al-2Zr-4Mo-2Sn alloy, Ti-4.5Al-3V-2Fe-2Mo alloy or the like.

[0018] Pure Titanium Material

[0019] As a pure titanium material as a melting raw material, spongetitanium lumps produced by the Kroll process can be used as a main rawmaterial. The present invention is not limited to the titanium sponge bythe Kroll process and pure titanium scrap which is generally availablealso can be used.

[0020] As scrap, for example, black scales which are produced ingrinding a surface portion of a slab of by melting an ingot producedfrom an A-class sponge titanium, white scales (also called “turnings”)which are produced in a sizing after forging thereof, cut pieces (alsocalled “chips”) which are produced in working of a rolled plate or baror wire can be used.

[0021] The pure titanium material which is used as a melting rawmaterial preferably contains 0.01 to 0.3 wt % of Fe, 0.003 to 0.03 wt %of N, 0.01 to 0.40 wt % of O, other inevitable components, and thebalance of Ti. The inevitable components may be not more than 0.05 wt %of Cr and Ni each, not more than 0.020 wt % of C, and not more than 100ppm of H, or the like.

[0022] The form of the pure titanium material described above may be aplate, bar, wire, or other form, and is not limited as long as thecompositions are within the ranges described above. However, the rawmaterial is preferably formed into a shape in which it is easy to formbriquettes. Specifically, the pure titanium material may preferably becrushed or cut into pieces having lengths of several centimeters.

[0023] Aluminum Master Alloy

[0024] As disclosed in Japanese Unexamined Patent ApplicationPublication No. 158955/92 described above, metal aluminum was suppliedalone to an EB melting furnace as the aluminum alloy componentconventionally. However, vaporizing loss of aluminum was substantialbecause of its high vapor pressure. In contrast, in the presentinvention, since the aluminum component is added in conditions of alloywith titanium, vaporizing loss is low. As the aluminum component,commercial products of alloy of titanium and aluminum can be used, andscrap materials of alloy of titanium-aluminum can be also used.

[0025] As scrap material which is generally available, Ti-6 wt % Al-4wt% V based materials are mainly used. In recent years, high-aluminumalloy based scraps such as the intermetallic compound of Ti-17 wt % Alor Ti-36 wt % Al can be used as scraps of titanium material for targets.These alloys are preferable for EB melting because vapor pressure ofmelting aluminum component is low. In addition, these alloys are hardand brittle due to high aluminum content. Therefore, crushing andgranulating process can be relatively easily performed to control thesize appropriate for melting.

[0026] Furthermore, the vapor pressure of these alloys is extremely lowcompared to metal aluminum, and vaporizing loss of aluminum can begreatly reduced. Therefore, variation of aluminum component in an ingotor variation of aluminum components among ingots can be reduced.

[0027] Melting Material for V

[0028] V for an alloy component has lower vapor pressure compared to Al,and vaporizing loss in EB melting will rarely be a problem. However, themelting point of V is 1890° C., which is higher than the melting pointof titanium, and it is effective to be added in conditions of the masteralloy.

[0029] As a master alloy of V, 35 wt % Al-65 wt % V alloy or 50 wt %Al-50wt % V alloy can be used, and alloys having desired composition canbe produced by adding predetermined amounts of such V master alloy.However, it is desirable that slightly more V be added than the desiredvalue because vaporizing loss of V is not zero.

[0030] Melting and Casting for Titanium Ingot

[0031] After the raw material described above is prepared to havepredetermined components, melting processes can be performed by using EBmelting furnace. The raw material for melting can be melted after beingformed into briquettes, or can be supplied as it is. The condition ofthe raw material is preferably chips rather than briquettes when Ti—Alalloy are used.

[0032] On the other hand, when generally available scraps are processedas a titanium-aluminum alloy, the scrap is preferably crushed andgranulated into predetermined size to be supplied. The ingot componentand grain size after melting can be uniform by performing suchpreliminary treatment. Specifically, it is desirable to be granulated ina range of from 4 to 20 mm.

[0033] There are drip melting methods and hearth melting methods in EBmelting. The drip melting method is a method in which raw material iscrushed and granulated into predetermined size and formed intobriquettes; an electron beam is irradiated to an end portion of thebriquette to melt it; and the melted portion is dripped into awater-cooled mold and solidified to obtain a titanium ingot. In thismethod, a process in which the melted raw material is formed intobriquettes beforehand is required.

[0034] On the other hand, in the hearth melting method mentioned above,a flat water-cooled copper mold called a hearth is provided before thewater-cooled mold described above, the melting raw material is suppliedto an upper space of the hearth while the electron beam is irradiated tomelt the raw material, and the melted material is dripped into thehearth mentioned above. A melted titanium bath is formed in the hearth,and this bath is forming a flow toward the water-cooled mold. HDIs (highdensity inclusions) contained in the raw material are settled andseparated to a bottom portion of the hearth while the melted rawmaterial is flowing in the titanium bath, whereby only clean titaniumbath flows into the water-cooled mold.

[0035] As explained above, melting pools must be maintained in both ofthe hearth and the mold, electric power cost tends to be higher comparedto the case of a drip melting method. However, pretreatment such asbriquette forming is not required in the hearth melting, granular rawmaterial can be used, and ingots of high quality can be obtained.

[0036] Both melting methods can be performed in the present invention,and the method can be selected according to the application of an ingot.For example, in the case in which extremely high quality andcharacteristics are not required in ingots, both the drip melting andthe hearth melting may be used. However, in the case in whichrequirements for ingot are strict, for example, in the case in whichinclusions such as HDIs must not be contained, such inclusion can beeffectively removed by performing the hearth melting.

EXAMPLES

[0037] The present invention is further explained in detail by way ofExamples.

Example 1

[0038] 965 kg of sponge titanium corresponding to Japanese IndustrialStandard 1, 2800 kg of Ti-6 wt % Al-4 wt % V alloy scrap, 75 kg of 35 wt% Al-65 wt % V alloy were prepared, and Ti-36 wt % Al alloy scrap ofintermetallic compound was used as the master alloy of aluminum.

[0039] Then, these materials were charged into an EB furnace of thehearth type and were melted in conditions as mentioned below, and Ti-6wt % Al-4 wt % V alloy was obtained. Compositions of each raw materialbefore melting are shown in Tables 1 to 4.

[0040] 1) Composition of Raw Material

[0041] 1. Titanium raw material: Sponge titanium corresponding toJapanese Industrial Standard 1 TABLE 1 Chemical composition Fe O NAnalyzed value (wt %) 0.034 0.043 0.005

[0042] 2. 6Al4V alloy raw material: Ti-6 wt % Al-4 wt % V alloy scrapTABLE 2 Chemical composition Al V Fe O Analyzed value (wt %) 6.20 4.150.15 0.20

[0043] 3. Raw material for Al: Ti-36 wt % Al alloy scrap TABLE 3Chemical composition Al Fe O Analyzed value (wt %) 36.0 0.10 0.20

[0044] 4. Raw material for V: 35 wt % Al-65 wt % V alloy TABLE 4Chemical composition Al V Fe O Analyzed value (wt %) 32.0 67.0 0.26 0.15

[0045] 2) Melting Condition

[0046] Degree of vacuum: 1×10⁻³ to 5×10⁻⁴ Torr

[0047] 3) Result of Melting

[0048] The compositions of the titanium alloy obtained by the methoddescribed above are shown in Table 5. As is obvious from Table 5, the Alcomponent of each titanium alloy is close to the desired value,furthermore, variation among ingots is small. TABLE 5 Analyzed valueMelting No. Al V Fe O Desired value 6.20 4.15 — — 1 6.20 4.15 0.15 0.132 6.18 4.16 0.14 0.15 3 6.22 4.14 0.16 0.14

Comparative Example 1

[0049] 1068 kg of sponge titanium corresponding to JIS 1 used in Example1, 2880 kg of Ti-6 wt % Al-4 wt % V alloy scrap, 75 kg of 35 wt % Al-65wt % V alloy scrap, and 57 kg of metal Al shot were prepared, thesematerials were charged into an EB melting furnace of the hearth type,and Ti-6 wt % Al-4 wt % V alloy was obtained with the same apparatus andmelting conditions as in Example 1. The analyzed value of titanium alloyingot obtained by melting is shown in Table 6. TABLE 6 Melting No. Al VFe O Desired value 6.20 4.15 — — 1 6.00 4.15 0.14 0.14 2 6.10 4.18 0.130.12

[0050] As is obvious from Table 6, vaporizing loss in the melting ofaluminum is great because the metal Al shot was used as aluminum rawmaterial in the Comparative Example. Therefore, the desired amount ofaluminum could not be obtained. Furthermore, the amount of aluminum ineach titanium alloy varied.

[0051] As explained above, Ti—Al alloy which is inexpensive and reliablein quality can be produced because titanium-aluminum alloy is preparedas a master alloy and this aluminum master alloy and pure titaniummaterial are melted by an electron beam to obtain titanium alloy.

What is claimed is:
 1. A process for production of titanium alloycomprising: preparing titanium-aluminum alloy as a master alloy, meltingthe aluminum master alloy and pure titanium material by electron beammelting to obtain a titanium alloy.
 2. The process for production oftitanium alloy according to claim 1, wherein the titanium-aluminum alloyis defined by a formula Ti_(x)Al, and x is a real number expressed by ⅓to
 3. 3. The process for production of titanium alloy according to claim1, wherein the titanium-aluminum alloy is obtained from scrap.
 4. Theprocess for production of titanium alloy according to claim 1, whereinthe titanium-aluminum alloy is a titanium-aluminum intermetalliccompound.
 5. The process for production of titanium alloy according toclaim 4, wherein the titanium-aluminum intermetallic compound is Ti₃Al,TiAl, TiAl₂, or TiAl₃.
 6. The process for production of titanium alloyaccording to claim 4, wherein the titanium-aluminum intermetalliccompound is obtained from scrap.